Activated carbon is an amorphous form of carbon which is specially treated to produce a very large surface area, ranging generally from 300 and 2,000 m.sup.2 /g. This large surface area means that the internal pore structure has been very highly developed. It is this structure that provides activated carbon with the ability to absorb gases and vapors from gases, and dissolved or dispersed substances from liquids. Activated carbons remove colors, odors and unwanted flavors from gases and solutions by adsorbing the undesired impurities on their surfaces. Activated carbon appears commercially in two forms: light fluffy powders generally used for decolorizing, and hard, dense granules or pellets generally used for vapor adsorption.
Almost any carbonaceous material of either animal, vegetable or mineral origin can be made into activated carbon when properly treated. Activated carbon has been prepared from the blood, flesh and bones of animals; it has been made of vegetable materials including hardwoods, softwoods, corncobs, kelp, coffee beans, rice hulls, fruit pits, nut shells (particularly peanut shells), bagasse and lignin; and it has been made of minerals including peat, lignite, soft and hard coals, tars, pitches, asphalt, petroleum residues, and carbon blacks. However, for economic reasons, bones, wood, peat, lignite and paper mill waste (lignin) are most generally used for the manufacture of powdered carbons, and cocoanut shells, coal, peat and petroleum residues are used for granular carbons.
Regardless of the raw material or the form of the product, activated carbon generally is made by one of two basic methods: chemical or gas activation. Chemical activation depends on the action of inorganic chemicals, e.g. zinc chloride or phosphoric acid, present naturally or added to the raw material to degrade or dehydrate the organic molecules during carbonization or calcination. Gas activation depends on selective oxidation of the carbonaceous matter with air at low temperature, or steam, carbon dioxide, flue gas, chlorine or similar gases or vapors at high temperature. The oxidation is usually preceded by a primary carbonization of the raw material.
The adsorbing power of an activated carbon varies directly with (i) the carbon purity of the activated material, and (ii) the surface area per unit of weight of the activated material. The higher the purity and the greater the bulk of a unit weight of activated carbon, the greater the adsorbing power. Granular carbons also require consideration of the resistance to flow of fluid or gas through a unit weight of the activated carbon. A granular activated carbon is provided in a packed bed, and the gas or liquid to be purified is passed through the packed bed. The more gas or liquid that can be passed through a unit weight of the activated carbon in a given time, and the more uniformly the gas or liquid can pass through the activated carbon, the greater is the absorbing power of the activated carbon. Another consideration for graunular activated carbons is that the granules have sufficient green and finished strength to permit automated processing through drying, carbonizing and activating furnaces and formation of the activated carbon bed.
Accordingly, methods for making activated carbon are directed to (i) decreasing the non-carbonaceous content, (ii) increasing the surface area per unit weight, and (iii) providing more uniformly sized and packed and spherically shaped granular carbon. For example, coal and lignite (e.g. brown coal) have been found to provide more pure activated carbon if first treated with hydrochloric acid to remove ash and then carbonized to remove the chlorinated hydrocarbons, e.g. U.S. Pat. No. 2,040,931 and Ind. Eng. Chem. 38, 7, 745 (1946). Similarly, coal has been processed to increase the surface area per unit of weight, e.g. U.S. Pat. No. 3,483,134, and processed with coal tar, coke-oven and pitch to form spherical granules of essentially uniform size, e.g. U.S. Pat. Nos. 1,478,986, 2,008,145, 3,533,961 and 3,623,999.
The present invention provides a simplified method of making an improved activated carbon from materials such as coal. It inexpensively provides activated carbons with high carbon content and high surface area per unit weight, and with uniform granular size and packing as well as generally spherical shape.